Inertial rotation sensors, for determining orientation and/or rate-of-turn with respect to an inertial frame of reference, are essential elements of attitude and heading reference systems used by navigable vehicles such as aircraft. (Hereafter, the expressions "with respect to" and "inertial frame of reference" are respectively abbreviated "WRT" and "IFR".) Since the beginning of this century, such determinations have typically been made using spinning mass gyroscopes. Progress in that art has resulted in many refinements and the development of various types of gyroscopes suited to specific applications. There now exists a large body of art related to them.
In spite of progress in the art of spinning mass gyroscopes, many limitations and problems still exist, and are particularly acute in applications requiring high accuracy. More specifically, these limitations and problems include: limited dynamic range (including limitations due to hysteresis and limited ability to measure high rates of rotation); slow reaction time; limited reliability (including susceptibility to shock damage); and, high cost (due to careful clean room assembly of precision complex machined components, etc.). These limitations, and others, are well known to those skilled in the art. See for example, Udd, E., 1985: "Fiberoptic vs Ring Laser Gyros: an Assessment of the Technology", Laser Focus/Electro-Optics December 1985, pp. 64-74.; and U.S. Pat. No. 4,675,820 issued Jun. 23, 1987 for an invention of Smith et al entitled "Inertial Reference System".
The above noted problems with spinning mass gyroscopes have in recent years prompted research and development of alternative forms of inertial rotation sensors. Such alternatives have included: devices exploiting the momentum of fluid streams; devices exploiting inertial forces acting on vibrating members; and, devices exploiting the tendency of various forms of waves to maintain their speed WRT an IFR. Generally, with the notable exception of `optical gyroscopes` which exploit the `Sagnac effect`, these alternatives to spinning mass gyroscopes have limited capabilities, especially in applications requiring high accuracy.
Attempts have also been made to exploit the tendency of a rotating mass to continue rotating at a constant rate WRT an IFR irrespective of changes in orientation of the axis of rotation and irrespective of rotations of supporting structure about the axis of rotation: U.S. Pat. No. 3,793,737 issued Feb. 26, 1974 for an invention of Alth entitled "Self-Timed Reaction-Mass Compass", and U.S. Pat. No. 4,706,389 issued Nov. 17, 1987 for an invention of Eisenhammer entitled "Attitude Displacement Measurement Apparatus" exemplify this approach. The present invention also utilizes this approach but implements it in a manner substantially different from the prior art.
As exemplified by the devices of Alth, Eisenhammer, and the present invention, devices exploiting the tendency of a rotating mass to continue rotating, have certain advantages for overcoming problems and limitations of other prior art. For these specific devices, which comprise rotatable masses rotating continuously about axes fixed to the supporting structure, without gimbals or balance assemblies: hysteresis is eliminated because of continuous motion; high rates of rotation are, in principle, readily measurable; reaction time on the order of one rotational period of the rotating mass is, in principle, achievable; and, susceptibility to shock damage and costs are reduced by a simple and robust mechanical configuration.
The devices of Alth and Eisenhammer, utilize pairs of coaxial members which are counter-rotatingly driven with respect to each other. In such a configuration, the friction torques opposing the motions of the two members act in opposite directions and, in principle, cancel each other to eliminate the net external friction torque acting on the pair of members. This configuration also isolates the driving torque from motions of the supporting structure.
U.S. Pat. No. 3,793,737 Alth discloses a device having a pair of coaxial counter-rotating members with each member pierced by a radial slot. Twice during each revolution, the slots line up to reveal positions on a compass card situated below the counter-rotating members and attached to the supporting structure. The positions revealed on the compass card indicate the orientation of the supporting structure. According to Alth, the slots tend to maintain their lined-up positions (WRT an IFR), regardless of motions of the supporting structure.
However, the slots can maintain their lined-up positions WRT an IFR only while the two members maintain precisely equal but opposite speeds of rotation WRT an IFR. If the speeds of the two members differ by any amount, the positions of the lined-up slots will drift in the direction of the faster member. Alth apparently relies on the inertia of the two members and the frictional balance provided by the counter-rotating members configuration but otherwise makes no provision for speed control. Consequently, drift in the lined-up positions of the slots is inevitable. Accordingly, Alth's device lacks the stability required of a precision instrument.
U.S. Pat. No. 4,706,389 Eisenhammer discloses a device having a pair of coaxial counter-rotating blades driven by a motor assembly within a base case. An arrangement of sensors is used for determining the motions of the blades WRT the case. Data-processing apparatus is provided to determine the difference in the speeds of the two blades WRT the case. If the case rotates about the axis of rotation of the blades, the difference in the speeds of the two blades WRT the case will change. The device is therefore sensitive to rate-of-turn of the case about the axis of rotation.
Although not explicitly stated, the effect of this arrangement is to tie the blades to the fluid within the case, through fluid friction. This fluid friction implicitly provides speed control for the blades. Unfortunately, the fluid friction is a function of unpredictable factors including the history of case motions. Because the blades remain frictionally attached to the fluid within the case and, because of their own momentum, they do not immediately follow rotations of the case. Therefore, changes in the speeds of the blades WRT the case can be used to determine changes in the rate-of-turn of the case WRT the IFR.
Eisenhammer does not provide a means for discriminating between slow changes in the speeds of the blades and slow drift in the orientation of the device. Thus, Eisenhammer's device also lacks the stability required of a precision instrument.
As described above, prior art devices which exploit the tendency of a spinning mass to continue spinning at a constant rate WRT an IFR, in order to overcome problems and limitations associated with other prior art, are known. However, it is evident that there remains a need for a better sensor which additionally provides stability and precision. The present invention provides stability and precision through implementing the approach in a manner substantially different from the prior art.